How did your friend get you to babysit her kids for the weekend, or your sister talk you into hosting the next book club meeting? They probably asked when you were anxious about a work project or stressed about making an impending mortgage payment.

Stress, however, isn’t traditionally associated with altruism. When self-discipline wanes, such as when you are hurried, hungry or distracted, you are less likely to be helpful to strangers (if you’re late for an appointment, you’re probably not stopping to help the person who just dropped the contents of his briefcase). That makes intuitive sense: helping someone you are unlikely to ever see again when you feel least in control of your own life isn’t likely to be productive.

Yet such selfishness seems at odds with the need for cooperation in a social species that relies on support from others for survival. So researchers have suspected that this pattern may only hold true for strangers— and that stress and periods when you feel your life is out of your control might actually increase sacrifice toward loved ones since collaboration with those upon whom you regularly rely is essential for survival.

“In communal relationships, the habitual behavior is to take care of each other’s needs,” says lead author Francesca Righetti, assistant professor of psychology at VU University in Amsterdam.

The study, which was published in Psychological Science, involved several experiments in which some participants were intentionally distracted by subtitles on a silent video, while others were not. The volunteers were then asked about sacrifices they would make for either a long-term significant other, or a best friend.

Those who had to cope with the distraction— a task known to reduce self-control— were more likely to say they would sacrifice for their friends or partners by going out with people they did not like (but whom their partners or friends liked) or by performing an embarrassing task for their loved one.

Righetti says that couples often face situations where their interests diverge and one must give up something for the other— like choosing whether to see one partner’s friends or deciding on a beach or mountain vacation.

“In these situations people need to choose between pursuing their own wishes and sacrificing to promote the well-being of their partner or relationship,” she says, “We found that when people are in a loving relationship, their impulsive response to these types of circumstances is to be nice and benefit their partner even if this is costly for them.”

The results confirm what most young children know intuitively— that if you want something from mom or dad, you should hit them when they’re distracted by something else. They also resolve what seemed to be a paradox: that good self control is associated with happiness and success, but could be poison for relationships by leading to selfish behavior. It seems there’s a balance between what we’re ready to give and what we take from our different interactions with people. Self-centeredness, these findings suggest, may apply primarily to strangers. When it comes to people we love, we’re willing to suffer.